Intrusion alarm system

Abstract

 An intrusion alarm system provided with a passive sensor with a detector for detecting energy of light (electromagnetic radiation) from an object in a location to be monitored, and with an alarm for generating an alarm signal, dependent on whether a detection signal is emitted by the detector or not, wherein an active sensor is provided, said sensor having a source for emitting light at least partially onto an aperture of the passive sensor and a detector for detecting reflected light from the source.

Description

[0001] The invention relates to an intrusion alarm system provided with a passive sensor with a detector for detecting energy of light (electromagnetic radiation) from an object in a location to be monitored, and with an alarm for generating an alarm signal, dependent on whether a detection signal is emitted by the detector or not.

[0002] Such an intrusion alarm system is known from European patent application No. 0 255 812 in the name of Elkron S.p.A. The intrusion alarm system described therein utilizes, in a manner well known, a passive infrared sensor, whereby infrared light emitted by an object in a location to be monitored is passed by optical means - via an aperture of a passive infrared sensor - to a detector in the shape of for example a pyro-electric element. The optical means can for example consist of a mirror or a Fresnel lens. An intruder in the location to be monitored is spotted as a result of the pyro-electric element detecting a change, generated by the intruder, in the amount of infrared light falling thereon and consequently activating an alarm, which alarm generates an alarm signal. In order to optimize the operation of the known intrusion alarm system the aforesaid European patent application proposes to couple the passive infrared sensor to a radio-frequency sensor operating in the UHF band. With the known intrusion alarm system an alarm signal is not monitored unless both the passive infrared sensor and the radio-frequency sensor - independently - detect an intruder in the location to be monitored. Whilst the detection operation of the passive infrared sensor is already outlined above, the detection operation of the radio-frequency sensor is in broad outline as follows. Movements made by an intruder in a location to be monitored, in which radio waves are emitted, cause a disturbance of the radio-frequency band (as a result of the Doppler effect), which disturbance is detected by the radio-frequency sensor, resulting in an alarm signal being generated.

[0003] Such an intrusion alarm system is also known from US patent No. 3,703,718 in the name of Herbert L. Berman. The infrared intrusion alarm system described therein utilizes a single passive sensor and optical means for focussing radiation directed at the passive sensor from various fields of vision in a location to be monitored. An amplifier, which is arranged so as to have a frequency response corresponding with the walking speed of an intruder, amplifies the signal from the passive sensor. The amplifier is provided with means for distinguishing between changes in the infrared radiation that are caused by the presence of an intruder and changes caused by gradual temperature changes, such as changes in the room temperature and the ambient temperature.

[0004] One drawback of the known intrusion alarm system is that it does not offer a solution for the following problem. Since the operation of the passive infrared sensor is based on the detection of infrared light, i.e. heat radiation with a wavelength in the order of in particular approx. 6 - 18 µm, emitted by an intruder in a location to be monitored, and since only very few materials possess good transmission characteristics for such infrared light (nearly all materials block, absorb and/or reflect this kind of light), the detection of the known intrusion alarm system can be easily sabotaged by placing materials that do possess good transmission characteristics for this kind of infrared light on and/or near the detector of the passive infrared sensor. When for example at least part of the aperture of the passive infrared sensor is blocked with materials such as paper, glass, paint, cardboard or plastic, the monitoring effect of the known intrusion alarm system is seriously affected (compromised). In some cases said affecting (sabotaging) of the quality of the known intrusion alarm system can be carried out without this being clearly visible to the user of the intrusion alarm system, whereby in a particular placing a glass plate in front of the detector of the passive infrared sensor or painting the window of the passive infrared sensor in a similar colour can be considered. A further drawback of the intrusion alarm system known from European patent application No. 0 255 812 is that it is complex and relatively costly, in particular owing to the use of two separate sensors, and that no alarm signal is generated unless both the passive infrared sensor and the radio-frequency sensor detect an intruder in the location to be monitored, so that, when one of the sensors does not function at all, or not optimally, no alarm signal is generated. The intrusion alarm system known from US patent No. 3,703,718 appears to be rather prone to sabotage in practice.

[0005] The object of the invention is to provide a simple and inexpensive intrusion alarm system, which makes it possible to detect sabotage to the passive sensor thereof.

[0006] For this purpose an intrusion alarm system of the kind mentioned in the introduction is according to the invention characterized in that an active sensor is provided, said sensor having at least one source for emitting light at least partially onto an aperture of the passive sensor and at least one detector for detecting reflected light from the source. Preferably the passive sensor is a passive infrared sensor and the active sensor is an active infrared sensor, based on the emission or detection of infrared light respectively. Thus an intrusion alarm system is provided which offers adequate security against sabotaging of the passive infrared sensor, such as approaching the passive infrared sensor with a hand, covering the sensor with a glass pane, approaching the sensor with white paper, covering the passive infrared sensor with cardboard, spraying the sensor with clear varnish and or covering the sensor with a foam plastic plate that absorbs infrared light.

[0007] It is noted that the intrusion alarm system according to the invention knows no restrictions with regard to the type of light being used, i.e. not only infrared light, but also visible light (for example with a wavelength between 0.35 to 0.8 µm) may be used. From a marketing point of view it may even be interesting to use visible blue, green or red light. Furthermore it is noted that an important advantage of the intrusion alarm system according to the invention is the fact that the active (whether or not infrared) sensor has a limited range, so that (sabotaging) manipulations on the aperture of the passive sensor and in the vicinity thereof are detected, whereas an authorized person when passing by the active sensor during the daytime does not generate an alarm signal.

[0008] One embodiment of an intrusion alarm system according to the invention is characterized in that an alarm is provided for generating an alarm signal in dependence on whether a detection is issued by the detector of the active infrared sensor or not. This alarm may be the alarm which is coupled, in an electromagnetic sense, to the passive infrared sensor, it may also be a separate alarm, however.

[0009] Another embodiment of an intrusion alarm system according to the invention is characterized in that the source can emit infrared light onto and around the aperture of the passive infrared sensor.

[0010] Another embodiment of an intrusion alarm system according to the invention is characterized in that the passive infrared sensor is sensitive to infrared light having a wavelength between 6 and 50 µm.

[0011] Another embodiment of an intrusion alarm system according to the invention is characterized in that the active sensor is sensitive to light having a wavelength between 0.35 and 4 µm.

[0012] Another embodiment of an intrusion alarm system according to the invention is characterized in that said source and said detector of the active infrared sensor at least substantially consist of a photoemitter and a photodiode respectively, both having an angle of opening between 60° and 120°.

[0013] It is noted that the intrusion alarm system according to the invention may include a passive infrared sensor coupled to a radio-frequency sensor, all this in accordance with European patent application No. 0 255 812 in the name of Elkron S.p.A. It is furthermore noted that with the intrusion alarm system according to the invention the active (whether or not infrared) sensor may also include more than one source (photoemitter) and/or more than one detector (photodiode). The specific advantage of this is that the alarm of the intrusion alarm system is not activated when for example insects come near the aperture of the passive sensor. It is to be preferred hereby that the sources and the associated detectors are sequentially driven in pairs.

[0014] The invention will be further explained with reference to Figures illustrated in a drawing, wherein:

Figure 1

shows a housing of a passive infrared sensor associated with a prior art intrusion alarm system;

Figure 2

shows the passive infrared sensor of Figure 1 in exploded view;

Figure 3

shows a housing of a passive infrared sensor associated with an intrusion alarm system according to the invention;

Figure 4

shows the housing of Figure 3, whereby infrared radiation as emitted or received by the source or the detector of the active infrared sensor respectively is drawn in full lines;

Figure 5

shows the housing of Figure 3, illustrating an area covered by the active infrared sensor (with conical envelopes of emitted and received beams of infrared radiation); and

Figure 6

schematically shows a block diagram of an electric circuit of an intrusion alarm system according to the invention.

[0015] In figure 1 a housing of a passive infrared sensor associated with a prior art intrusion alarm system is shown, said housing including a window 1 for the detector of the passive infrared sensor, a cover 2, a mounting base 3, an alarm light 4 which will light up when the alarm is activated and means of attachment 5 for securing the cover 2 and the mounting base 3 together.

[0016] figure 2 shows the passive infrared sensor of figure 1 in disassembled condition, whereby besides the aforesaid parts also the following parts are depicted: an insulation plate 6, a sticker 7 with connection data, an insulation sticker 8, a metal radio-frequency shield 9, an amplifier circuit board 10, a pyro-electric element 11 with a holding fixture, a circuit board 12 with control functions, a far infrared focusing mirror 13, a masking plate 14 for long detection fields, a type-indication sticker 15 and masking plates 16 for short detection fields.

[0017] In figure 3 a housing of a passive infrared sensor associated with an intrusion alarm system according to the invention is depicted. Said housing includes a window 1 for the detector of the passive infrared sensor, a cover 2, a mounting base 3, an alarm light 4, means of attachment 5 for securing the cover 2 and the mounting base 3 together, windows 17 and 18 for the source and the detector of the active infrared sensor respectively, and an alarm light 19 which lights up when it is attempted to sabotage the passive infrared sensor. It is noted that from a purely technical point of view the windows 17 and 18 are not absolutely necessary, but in principle function to make the unit look more attractive.

[0018] In figure 4 the infrared radiation emitted by the source 20 or received by the detector 21 of the active infrared sensor respectively is illustrated in full lines 22.

[0019] Figure 5 shows the area covered by the active infrared sensor with conical envelopes 23 of emitted and received beams of infrared radiation. Those parts in figures 4 and 5 that correspond with parts shown in figure 3 are indicated by the same reference numerals.

[0020] The operation of the intrusion alarm system according to the invention will be explained in more detail with reference to the block diagram of figure 6 of an electric circuit of said intrusion alarm system.

[0021] The source of the active infrared sensor consists of a photoemitter 24 (near infrared transmitter (NIR-TX)) having an angle of opening between 60° and 120°, said photoemitter 24 emitting radiation in the near infrared wave range onto and around the aperture of the passive infrared sensor (see the preceding figures). The passive infrared sensor is actually sensitive to infrared light of the far infrared wave range. The photoemitter 24 is connected to a power driver amplifier 25, which generates pulse flows with peak currents in the order of about 1 A, so that the photoemitter 24 emits short infrared pulses at a high intensity. A master oscillator 26, with a pulse repetition time in the millisecond range and a pulse time in the microsecond range, provides the timing of the photoemitter 24. The aforesaid window of the passive infrared sensor is possibly covered with a (very) fine texture, in such a manner that infrared light from the photoemitter 24 that falls thereon is scattered in numerous directions. The advantage of this is that a certain amount of background light is received by the detector of the active infrared sensor at all times, so that a "reference reflection signal of a constant low value" is present at all times. For the same reason objects may be placed (as a testing method) near the passive infrared sensor, in order to effect that the scattered light is directly passed from the photoemitter 24 to the detector of the active infrared sensor.

[0022] The detector of the active infrared sensor consists of a photodiode 27 (near infrared receiver (NIR-RX)), likewise with an angle of opening between 60° and 120°, which photodiode 27 is receptive to infrared light scattered by the window and adjacent wings (indicated at V in figure 3) of the passive infrared sensor, as well as by objects located in the immediate vicinity of the window. The photodiode is connected to an amplifier/filter 28, which amplifies pulses at a high rate and which rejects signals having a low frequency, such as signals caused by fluctuations in the ambient light. A peak detector 29 detects the peak amplitude of the fast infrared pulses received by the photodiode 27 and amplified by the amplifier/filter 28. In this connection it is noted that a system of transmitting and amplifying short infrared pulses with a high intensity has been opted for, on the one hand in order to conserve energy and on the other hand in order to retain the possibility of distinguishing the pulses emitted by the photoemitter 24 from fluctuations in the ambient light. The peak detector 29 is followed by a band pass filter 30 which only amplifies variations in the peak amplitude ranging from slow to very slow (0.001 - 1 Hz). This was opted for in order to filter out ultra-slow amplitude variations, such as caused in particular by ageing of used semiconductors or by thermal drift, and in order to keep detecting in a reliable manner the slow movement of objects towards the window of the passive infrared sensor during an attempt at sabotage. The peak detector 29 may be synchronized by means of the master oscillator 26. As a result of the addition of a synchronisation signal the peak detector 29 will only be operative for a short time, during which also a transmission pulse of the photoemitter 24 takes place. As a result of this the signal-noise ratio of the intrusion alarm system according to the invention will be improved considerably. The following improvements will be possible in that case: a. a greater immunity to daylight (the system continues to operate in a reliable manner, even with direct incident sunlight), b. a much smaller consumption of emitter current and yet an adequate functionality, and c. greater reliability and a longer life of the intrusion alarm system due to the reduced load of active semiconductor devices. A window comparator 31 with a logic alarm circuit connected thereto, which is linked to the band pass filter 30, is activated when predetermined limiting values are exceeded, which indicate that the quality of the intrusion alarm system according to the invention is affected as a result of an attempt at sabotage. A low limiting value indicates that there is less scattering of infrared light in the direction of the photodiode 27. This points for example to changes with regard to the scattering by the aforesaid fine texture or by the aforesaid wings, which may be caused by varnish or paint being sprayed on the window of the passive infrared sensor. This will also be the case when the windows of the photoemitter 24 and the photodiode 27 are covered or when the photoemitter 24 or the photodiode 27 does not function optimally. A high limiting value indicates that a reflecting object must be present in the vicinity of the window, which object increases the amount of infrared light travelling from the photoemitter 24 to the photodiode 27. This will inter alia be the case when a glass pane is used to cover the detector of the passive infrared sensor or when an intruder attempts to cover the aperture of the passive infrared sensor with his hands, a sheet of paper or a piece of plastic.

[0023] The sensitivity of the intrusion alarm system according to the invention with regard to the detection of reflecting materials, absorbent materials and attempts at spraying paint can be optimized by

• placing the photoemitter 24 and the photodiode 27 at an acute angle, in particular an angle of less than 20°, with respect to the window of the passive infrared sensor;
• optimizing the characteristics of the (very) fine texture on the window of the passive infrared sensor, so that light scattered therefrom can be optimally transmitted to the photodiode 27;
• using more than one path along which infrared light can travel from the photoemitter 24 to the photodiode 27, especially by introducing wings V (see figure 3).

Claims

1. An intrusion alarm system provided with a passive sensor with a detector for detecting energy of light (electromagnetic radiation) from an object in a location to be monitored, and with an alarm for generating an alarm signal, dependent on whether a detection signal is emitted by the detector or not, characterized in that an active sensor is provided, said sensor having at least one source for emitting light at least partially onto an aperture of the passive sensor and at least one detector for detecting reflected light from the source.

2. An intrusion alarm system according to claim 1, characterized in that the passive sensor is a passive infrared sensor and the active sensor is an active infrared sensor, based on the emission or detection of infrared light respectively.

3. An intrusion alarm system according to claim 2, characterized in that an alarm is provided for generating an alarm signal in dependence on whether a detection is issued by the detector of the active infrared sensor or not.

4. An intrusion alarm system according to claim 2 or 3, characterized in that said source can emit infrared light onto and around the aperture of the passive infrared sensor.

5. An intrusion alarm system according to claim 2, 3, or 4, characterized in that said passive infrared sensor is sensitive to infrared light having a wavelength between 6 and 50 µm.

6. An intrusion alarm system according to any one of the preceding claims, characterized in that said active sensor is sensitive to light having a wavelength between 0.35 and 4 µm.

7. An intrusion alarm system according to any one of the preceding claims, characterized in that said source and said detector of the active infrared sensor at least substantially consist of a photoemitter or a photodiode respectively, both having an angle of opening between 60° and 120°.

Drawing